A toner used for an electrophotographic system, especially, an electrophotographic system using a heat roller fixing system, has been desired to have excellent low-temperature fixing ability and offset resistance. In view of the above, JP2000-35695 A discloses that a polyester resin obtained by using an alkylsuccinic acid, an alkenylsuccinic acid, or a mixture thereof, each having 10 or more carbon atoms, is excellent in low-temperature fixing ability and offset resistance.